Photo Basics – Metering

In the first three instalments of the Photo Basics series we looked at the ‘exposure triumvirate’. That’s the combination of aperture, shutter speed and ISO setting that give us the overall exposure of our photo. But how is that exposure determined? With a light meter.

All modern cameras have built in light meters. Even the entry level, fully automatic point & shoot cameras have a light meter. The sophistication of these meters varies. SLR-type cameras have very sophisticated meters.

There are two types of light meters – reflected and incident.  Most of us tend to use the meter in our camera which is a reflected light meter.  It measures the light that is bounced off or reflected by the subject or scene.  An incident light meter measures the light that falls on the subject.  Hand held light meters often have both functions.

In the Zone System, Ansel Adams would use a handheld spot meter. These are specialised, reflected light meters that measure from a very small area – typically a 1 degree arc. Because the measurement area of these meters is so small, very precise measurements of different areas of a scene can be made without overlap of one brightness zone into another. Most standard handheld meters don’t have a spot function. If such a feature exists, it will be stated as such.

Some in camera meters also have a spot feature but it’s not quite as useful as a handheld spot meter. Most in camera spot meters measure a percentage of the field of view. Some as small as 1%, others 3% and some as much as 9%, which really isn’t a spot meter at all since it’s too wide a measurement area.

On a wide angle lens, even a 1% spot meter can be measuring a wider area than is desired. If the lens has a 140 degree angle of view, a 1% spot meter will be measuring 1.4 degrees of arc. The area of measurement is tied to the angle of view of the lens. Longer lenses with narrower fields of view will mean a smaller measurement area.

Most DSLR cameras have there or four different metering modes. They can have a spot meter, as mentioned above. Most will have a centre-weighted average meter. Many will have an average meter. And today, pretty much all will have an advanced multi-segment meter.

All of these are reflected light meters but work in slightly different ways. We’ve already discussed how the spot meter works. The centre-weighted average meter works by averaging the brightness across the scene in the viewfinder but giving more weight to the centre of the frame because a lot of people centre their subjects.

The average meter works by simply averaging the different areas of brightness in the viewfinder to come up with an average brightness reading and setting an exposure for that average brightness.

The advanced, multi-segment meters are a high tech average meter, if you will. Modern cameras have light meters that have many different areas from which the camera will take readings. Each of these is read discretely by the camera’s on board metering computer. The active focus point(s) are also taken into consideration to help determine how the weighting of the different readings will be done with the active focus point(s) being used as a higher area of importance due to the built-in assumption that the main subject is at that point. So the computer will calculate an average meter reading by weighting the presumed area where the subject is more heavily and other areas around that progressively less. These multi-segment meters are generally very ‘intelligent’ and accurate and do come up with a good exposure setting in most circumstances. Like all reflected light meters; however, they can be fooled.

Fooled? How? In addition to the Zone System, Ansel Adams developed the standard for light meters. He determined the standard would be that all reflected light meters would calibrate to a ‘middle grey’ tone. Middle grey happens to be Zone V in the Zone System and represents an amount of 18% of the light that hits the subject is reflected back to the meter. This is why we have 18% grey cards for taking meter readings.

What this means is that the meter wants to turn everything it ‘sees’ into a middle grey tone. If you take a photo of a black cat in a coal bin what colour do you think it will come out as? If you were to take a picture of a white sheet laying on snow, what colour do you think it would come out as? The answer is the same in both cases – middle grey. The meter is fooled by overly light or overly dark subjects and comes up with the wrong exposure as a result.

The examples below will help illustrate. The shots are of a hibiscus plant in my garden. The petals are white with contrasting dark pink areas. In the first image, I took the shot using my camera’s spot meter measuring off the white of the petal.

2 stops underexposed

The white petal is middle grey. If we look at the histogram for the image it shows us the same thing.

Meter Reading - Two Stops Underexposed

The distribution of tones in the image is skewed far to the left indicating an overly dark image. The in camera meter turned our white petaled flower into a grey petaled flower.

If we increase the exposure of the image by 1 stop what do we get?

+1 Exposure Compensation

The petals are a much lighter grey, but still not white. What’s the histogram look like?

+1 Exposure Compensation

The distribution of tones on the histogram chart is pushed more to the right but still not as far as it needs to be for a correct exposure.

Let’s look at what increasing exposure by two stops does.

+2 Stops Exposure Compensation

Now we have nice, white petals on our hibiscus. Just what we want. And the histogram?

+2 Stops Exposure Compensation

Now the tones are pushed to the right and we have some whites right at the edge of the graph, but no clipping. That is, there isn’t a spike at the very right edge of the graph.

If we did the same experiment with a predominantly black subject, we’d get the same result but in reverse.  The black would turn out middle grey at the meter reading and by dialing in negative exposure compensation (i.e., reducing exposure) we’d get back to the proper black subject with about 2 stops of compensation.

When we’re using the reflected light meters in our cameras to determine our exposure settings, we need to know that while these meters are very sophisticated and generally very accurate, there are times when they can be fooled.  Understanding the situations when this can happen is crucial to getting a proper exposure and not ending up being disappointed when we get back home and begin looking at our pictures.  Also understanding the functions of our cameras and how to set exposure compensation when working in Aperture Value or Shutter Value is important.  In Manual mode, of course, we just adjust the shutter speed and/or aperture directly.  Unfortunately, the more automated P&S cameras don’t give us this level of control so we’re stuck with what the meter gives us in those cases.

The same is not true for handheld meters measuring incident light.  By measuring the light falling on the subject, there isn’t the same possibility of the meter being fooled so if measuring incident light, we can simply set our cameras to the exposure indicated by the incident meter and shoot away.  Using incident meters isn’t always possible; however, so we do need to know the limitations of reflected light meters.

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